It has been common practise to feed cooling air from an intermediate stage of the turbine compressor to various "hot spots" in gas turbine engines to provide the necessary cooling. This has been done traditionally by means of hollow pillars or by passing the cooling air through hollow blades or vanes in the turbine. Both these methods have drawbacks in that the hollow pillars tend to interrupt the flow of hot gasses and generally lower the turbine efficiency. Turbulence may result which may lead to blade vibration and shortened blade life.
Feeding cooling air through hollow stationary blades may set up thermal stresses in the turbine structure which may cause twisting or warping which can lead to premature failure of the stressed components in the machine. In particular any bearings in the combustor region of the turbine are subject to overheating and some means must be found to cool bearings in the combustor region. Similarly in the turbine under question some means must be found to cool the parts of the turbine in the region of the split-shaft or severe overheating will occur.